Digital thermostats need power. Operating power is typically provided from battery or from the thermostat wiring. A typical HVAC system runs on low voltage 24 VAC system and has a 110/220 VAC to 24 VAC transformer. The two sides of the transformer are typically marked as R (Return) and C (Common). Newer house wirings routes both taps of the transformer to the thermostat and thus the thermostat has direct access to this 24 VAC system and can derive its required internal supply voltages from the 24 VAC directly.
However, older houses do not typically have the C wire routed to the thermostat. Instead the C side of the terminal is routed through various demand controls, such as Fan, Heat, Cool, etc. The thermostat activates a relay and shorts these connections to the R, thus signaling a demand. When the contacts of the relays are open, the full 24 VAC is available between the various demand lines and the R. When the contacts are closed, the voltage drops to 0 VAC and the current flows from the C terminal of the 24 VAC transformer via the demand wires back to the R terminal of the transformer.
There have been on the market various power stealing methods that allow stealing power from these demand wires when the relay is open (voltage driven) and even when the relay is closed. The problem with these solutions is that they only allow a ‘small’ amount of power to be harvested, because if the current increases above approximately 10 mA or so in the demand line, the HVAC controller might detect a false demand on the control line. Most digital thermostats are very low power and may survive on this small amount of power harvested from one or more control lines. They may also be supported with battery backup and power stealing may be used just extend the battery life. There is also a solution that steals power from systems with a single demand line when the demand is not active, storing some of the energy in a rechargeable battery or super capacitor, and then powers the thermostat from this battery when the demand is active.
Newer thermostats are now getting network attached. Some network attached thermostats use a wireless interface and nowadays Wi-Fi is popular. The problem with a Wi-Fi attached thermostat is that it needs more power than can be stolen from an HVAC system without the C terminal. Thus this thermostat either requires the presence of the C wire or requires an external wall mount power supply.
What is needed is a system and method for powering a digital thermostat in the absence of an external power source such as a C wire or an external power supply.